Field of the Invention
The invention relates to a measuring arrangement.
Background of the Invention
Level gauges and methods for determining the fill level of a filled or bulk good in a container are known from prior art and are used, for example, in order to detect if a fill level is above or below a predetermined fill status. Such devices are also frequently called fill limit switches.
Fill limit switches can be based on very different physical principles. One of these principles is the evaluation of a capacity, which forms between a measuring electrode and a reference electrode, with the fill good during the filling process of the container fills a space between the reference electrode and the measuring electrode, and here changes the capacity formed by them. There are several different methods to measure the capacity dependent on said fill level. One method for determining the capacity is the formation of an electric resonance circuit including the capacity in question. The resonance frequency of the resonance circuit characterizes here the size of the capacity to be measured.
The resonance circuit may be embodied for example as a serial oscillating circuit with a serial switch of an inductivity with a capacity.
Another principle for a fill level switch comprises the use of a conductor resonance. A conductor embodied as an oblong probe generates reflections of an electric alternating voltage supplied, from which a standing voltage oscillation forms on the probe. The voltage, which can be measured at the voltage feed point, shows a resonant behavior due to the standing oscillation, with here the length of the conductor determining the resonance points. When the conductor probe contacts the fill good, by the capacities and/or additional reflections said fill good causes changes to develop in the frequency position of the resonance points. These can be evaluated with regards to a fill level reported.
The excitation of an electric resonance circuit requires the feeding of an alternating power signal via an alternating power generator. The resonance frequency can be found by changing the frequency of the signal supplied. It is characterized either by elevated voltage or by minimum voltage. The detection of the amplitude of the alternating voltage over the frequency allows here the determination of the resonance point(s).
Some applications of these principles of the limit detection of fill good require for example the installation of the sensor from the top through an existing opening in the lid of the container. However, if simultaneously the switching point of the limit switch shall not be positioned in the upper area of the container, thus in the proximity of the container lid, but further down, the detecting electrode or probe must be moved via an extension to the desired height of the container. Here it is advantageous if the sensor electronic can be fastened outside the container, in this case therefore above the lid of the container. This leads to the fact that the probe has to be separated from the electronic via an extension. This separation is also required in case of applications with very high temperatures inside the container, so that the electronic can be operated without problems distanced from the probe and the temperatures given there.
This leads to the requirement that a conductor is required between the electronic with the generator for the alternating power and the detector of the alternating voltage on the one side and the probe detecting the limit with the electric resonance circuit on the other side, connecting these two components. The electric conductor inserted here leads to the formation of line resonances on the connecting conductor. These line resonances develop in addition to the resonances of the probe, intended by the measuring principle and important for the function thereof, and can lead to disturbing interferences. Such line resonances are particularly disturbing when they are in the same frequency range as the resonances of the probe.
Therefore attention is paid in prior art that the length of the conductor of this extension line is so short that its lowest resonance frequency, thus for example the λ/4-line resonance, is still clearly above the resonance frequency of the probe.
With regards to the functional safety of the measuring principle, even when media adhere to the probe, it is advantageous to select resonance frequencies of the probe as high as possible, preferably higher than 10 MHz, more preferably higher than 50 MHz and even more preferably in a range higher than 100 MHz.
Accordingly, here a conflict develops between the selection of the resonance frequency of the probe and the potential conductor length of a desired extension between the probe and the electronic. In case of existing devices, this leads for example to the fact that only short extensions of less than 50 cm can be offered.
This problem shall be attained with the present invention. The object of the invention is to provide a level gauge, particularly a fill level switch, according to prior art which allows a spatial arrangement of the probe and the electronic showing a distance as long as possible.
This objective is attained in an arrangement showing the features described herein.